The present invention relates to a method of measuring, in a noncontact manner using laser light, the amount of elongation/contraction of, for example, a specimen mounted on a material testing machine, and also relates to a laser noncontact extensometer as apparatus which embodies the method above-mentioned.
The following methods are known as examples of the technique of measuring, in a noncontact manner, the amount of elongation (the amount of contraction; this also applies to the following description) of a specimen with the use of reflected light obtained by irradiating laser light onto the surface of the specimen: a method using speckle patterns, a method using a laser Doppler effect and a method in which marks are attached to two parts of the surface of a specimen, the positions of the marks are measured using laser light and the distance between the two marks is calculated from moment to moment.
According to the method using speckle patterns, a laser spotlight is irradiated onto a specimen at a single irradiation position which is a fixed and predetermined position, and scattering light reflected from the specimen surface is measured at each of two different positions, thus measuring two speckle patterns. Then, the cross-correlation functions of the speckle patterns are obtained such that the amounts of movement of the speckle patterns are calculated. Then, the difference between these movement amounts is obtained, and there are cancelled each other the pieces of information relating to the amounts of translation contained in the calculated results of speckle pattern movement amount. Thus, only the amount of distortion in the laser spot remains (Japanese Patent Publication No. 61-27681).
According to the method using a Doppler effect, the amount of elongation of a specimen between two parts thereof is calculated based on the following principle. Two laser beams respectively having different wavelengths are irradiated, as forming a certain angle, onto a specimen at a single stationary and predetermined point, thus producing interference fringes as modulated by a difference in frequency between the two laser beams. When the specimen is moved, the Doppler effect causes the modulated frequency of the interference fringes to be shifted by an amount corresponding to the moving speed of the specimen. With the use of this phenomenon, the moving speed can be obtained and then subjected to time quadrature such that the movement amount of the specimen at the laser beam irradiation position can be obtained. When, with the use of this principle, the movement amounts of the specimen at two parts thereof are obtained and the difference between these movement amounts thus obtained is calculated, the amount of elongation of the specimen between these two parts can be obtained.
According to the method of measuring the positions of two marks attached to a specimen with the use of laser light, laser light in the form of a spotlight is irradiated to a specimen while the specimen is being scanned by the laser light in the elongation direction with the use of a polygon mirror or the like, and the intensity of the reflected light is continuously detected such that the positions of the two marks are obtained from moment to moment. Based on the variations of the mark positions, the elongation of the specimen between the two marks is obtained.
The inventor of the present invention has proposed two techniques using speckle patterns, i.e., 1 one technique by which the amounts of movement of speckle patterns at two parts of a specimen are obtained, the difference therebetween is then calculated, the pieces of distortion information contained in the respective movement amount data are cancelled each other and the amount of elongation or contraction of the specimen between the two parts is calculated, and 2 the other technique by which, with the use of calculation results of movement amount of two parts of a specimen as calculated with the use of such speckle patterns or a laser Doppler effect, two laser irradiation positions are changed as following the deformation of a specimen such that the elongation of the specimen between two parts thereof is measured (Japanese Patent Laid-Open Publication 7-4928).
Out of the methods mentioned earlier, according to the method by which an amount of distortion is calculated based on two speckle patterns obtained by irradiating laser light onto a specimen at a single irradiation position, the obtained amount of elongation is limited to that of the specimen within the laser spot. Accordingly, even though a crack or the like has been produced on other parts of the specimen, such information is not reflected on the elongation data. Further, such elongation is completely different in concept from the elongation in accordance with JIS using a conventional contact-type mechanical extensometer or the like.
On the other hand, the situations of the broad zone between two parts of the specimen are reflected on the data of elongation obtained (i) according to the method 1 which has been proposed by the inventor of the present invention and by which the movement amounts of speckle patterns at two parts of a specimen are obtained and the difference therebetween is then obtained, or (ii) according to the method by which, with the use of a laser Doppler effect, the movement amounts of a specimen at two parts thereof are obtained based on the moving speeds thereof and, according to the difference between the movement amounts thus obtained, the elongation of the specimen between the two parts thereof is obtained.
However, the elongation obtained based on the movement amounts of speckle patterns at two parts of a specimen or based on the movement amounts of two parts of a specimen using a Doppler effect, is different in concept from the elongation determined by JIS in that the laser irradiation positions undergo no change from the beginning. More specifically, when the distance between two laser irradiation positions is set to GL, the elongation obtained with the two laser irradiation positions fixed, refers to an integrated value of the movement amount of a specimen which has crossed two observation points fixed with the distance GL provided therebetween. On the other hand, the elongation stipulated in JIS tensile test, refers to a rate or amount of change which represents how the distance GL between the initially set two points (gage points) undergoes a change after the test has been conducted. Thus, both the elongations are not equivalent to each other. Further, according to the method by which the elongation of a specimen between two points is calculated without the laser irradiation positions changed, there is the likelihood that, when the elongation is large, the initially set two points deviate from the observation points due to the elongation of the specimen by the test, such that no accurate measurement is made.
It is the technique 2 proposed by the inventor that has solved the problems above-mentioned. According to this technique, the laser light irradiation positions are changed to follow initially set two gage points according to the measurement results of movement amount, from moment to moment, of the gage points. It is therefore possible to obtain a measurement result of elongation similar to that obtained with a conventional mechanical contact-type extensometer. Further, by enlarging the range in which the laser light irradiation positions are moved, the technique 2 can cope with a large elongation of a specimen.
However, this following method requires a complicated mechanism for changing the laser light irradiation positions. This is disadvantageous in that the apparatus is increased in cost and that errors are contained in a measurement result due to the errors of the mechanism.
On the other hand, according to the method by which the positions of two marks attached to a specimen are measured by laser light, the problems above-mentioned are not encountered. However, this method is disadvantageous in that a lot of labor is taken for attaching the marks and that a mark possibly comes off with the advance of the test.